Radioactive heat source and container with helium permeable window in the container



May 12, 1964 R. s. ROCHLIN 3,133,196

RADIOACTIVE HEAT SOURCE AND CONTAINER WITH HELIUM PERMEABLE WINDOW IN THE CONTAINER Filed 001;. 29, 1959 O 7271p c 800 400 200 I00 *2! 0 "25 -78 Fig. 2.

Ti /a Inventor.- Robert Sfioch/in,

His Attof'ney.

United States Patent Oflice 3,133,196 RADIOACTIVE HEAT SOURCE AND CONTAINER WiTH I-IELIUM PERMEABLE WINDOW IN THE CONTAINER Robert S. Rochlin, Schenectady, N.Y., assignor to General Electric Company, a corporation of New York Filed Oct. 29, 1959, Ser. No. 849,593 4 Claims. (Cl. 250-106) This invention relates to a novel heat generating apparatus and the manner of fabricating such an apparatus. More particularly, this invention deals with a heat generating apparatus in which a radioactive isotope is utilized as the heat generating element.

A continuing need exists for new, compact, high output energy sources to replace such presently available storage devices as chemical batteries, etc. The search for such new devices has been accelerated by the severe size and output requirements for power sources in space vehicles. One solution which has been proposed contemplates utilizing a nuclear battery in the place of the presently known chemical batteries. Such a nuclear battery, in one simple form, comprises a sealed capsule containing a radioactive isotope which in decaying generates heat and the heat is then converted to electrical energy by means of an energy conversion device such as a thermocouple or a thermionic converter.

From many standpoints, such as efficiency, size, and particularly safety, it is preferable to utilize an alphaparticle-emitting isotope in the radioactive heat source. Alpha-emitting isotopes do, however, present severe encapsulation problems for the reason that as-the isotope decays the alpha particles, which are helium nuclei, cause a continuous buildup of helium pressure within the capsule. The danger is, therefore, always present that the helium pressure may become sufficiently great to rupture the capsule, scattering the radioisotope and contaminating the environment.

A number of solutions for avoiding dangerous pressure buildups in the capsule have been proposed. However, none of the solutions are completely satisfactory since they either fail to solve the problem or, in solving it, introduce additional complications and difficulties. It has been suggested, for example, that a large empty space be provided within the capsule to act as a gas reservoir and reduce the strain on the capsule. It is apparent that such a construction increases the size and overall bulk of the heat source. In space vehicles or the like, size and weight are serious problems, and any construction which does not minimize size and weight is undesirable. In addition, such a construction suffers from a further disadvantage because the inclusions of such a gas reservoir space substantially decreases the overall efl'iciency. The capsule or container for the heat-generating radioisotope must transfer heat to the converting device to produce electrical power from the heat energy. Since the radioisotope in the capsule is in a molten condition, the empty space interferes with optimum heat transfer by permitting the molten isotope to move freely in the capsule under zero gravity conditions, which are quite likely in a space vehicle satellite. The reduction or interference with the optimum heat transfer will, of course, have an adverse and highly undesirable effect on the electrical output from the nuclear battery.

It has also been suggested that the helium pressure inside of the capsule may be relieved by venting it through a small opening in the capsule provided for that purpose. Such a construction is not feasible since the radioactive isotope, either in powder, liquid or vapor form would escape from the capsule, contaminating the entire environment and producing a serious health hazard.

It is a primary object of this invention, therefore, to pro- 3,133,196 Patented May 12, 1964 vide a radioactive heat source which is compact and sturdy, and in which gas pressures on the container are relieved without allowing any of the radioactive material to escape.

A further object of this invention is to provide a sealed radioisotope heat source constructed to be impervious to the radioisotope in any physical form and permeable to helium gas.

Other objects and advantages will become apparent as the description of the invention proceeds.

The foregoing objects are achieved by constructing a large sealed radioactive source containing a radioisotope for generating heat. The housing of the source is so constructed that at least one portion thereof is constituted of a material through which the helium permeates to relieve the pressure, and which is impervious to the radioisotope, whether in its solid, liquid, or gaseous form. This may be achieved by utilizing a material, such as fused quartz, for example, which is permeable on the basis of atomic number and atomic size rather than on the basis of physical state, i.e., solid or gaseous. In this fashion, radioactive gases, such as radon and thoron, do not permeate through the material, whereas the helium generated by the emission of alpha particles is free to do so.

Some of the novel features which are believed to be characteristic of this invention are set forth with particularly in the appended claims. The invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following descriptions taken in connection with the accompanying drawings, in which:

FIG. 1 represents an embodiment of a sealed heat source constructed in accordance with the invention.

FIGS. 2 and 3 are curves illustrating the operating characteristics of the helium permeable element of the sealed source of FIG. 1;

FIG. 4 is an isometric perspective of an alternative embodiment of the heat source; and

FIG. 5 is a section taken along the line 5-5 of FIG. 4.

Referring now to FIG. 1 of the drawings, a sealed radioisotope source constructed in accordance with the invention is shown and includes an open ended cylindrical housing 1, preferably fabricated of stainless steel or other similar material, which contains the radioactive alphaemitting isotope 2, in molten form. The isotope 2 fills the entire container. The housing 1 is closed by means of a cap or plug 3, and is sealed against leakage by welding the cap to the housing 1 as at 4. A nickel coating seal 5 surrounds the housing 1 and a portion of the cap 3, and further insures against the escape of the alphaemitting isotope 2.

The nickel coating may be produced by placing the container, a portion of which is masked, in an atmosphere of nickel carbonyl and heating it by induction to the desired temperature. In this manner, a coating of .02 inch of nickel may easily be deposited on the unmasked portion of the surface to prevent the escape of the radioactive material from the interior of the housing 1. Other coating materials or other coating thicknesses may be employed, since neither the material nor the thickness of the coating is critical as long as the container is sealed. Container 1 as well as the plug or cap 3 should be fabricated from materials which will not combine chemically with the isotope 2. In the instant case, since the alpha-emitting isotope is Polonium-ZIO, it is preferable to fabricate the container 1 from stainless steel, although other suitable materials such as wrought iron, etc., may be utilized.

The choice of the particular alpha-emitting isotope which may be used in the novel sealed heat source, as well as its physical form, depends on a number of different considerations such as rate of decay (half life), ease of shielding penetrating radiation, weight per watt,

Isotope Po Am Half T11. f

138 Days 458 Years. 5.3 5.46.

It is apparent from the above tabulation that the Polonium-210 isotope produces a much greater amount of heat for a given volume and weight of the isotope, and hence would be preferable where size and space considerations are important. However, Americium-24l as well as other alpha-emitters are suitable for this and other utilizations.

Polonium-ZlO may be introduced into the container 1 in various physical forms and in many different ways. In the preferred embodiment, the polonium is shown in the molten form, and is introduced into the container before sealing as a liquid. The heat generated by the polonium in the course of its decay will then maintain the isotope in its molten condition. Alternatively, the poloniurn may be introduced in powder or in solid form. In fact, the polonium may be introduced as a powder and, depending on the total physical size of the isotope, enough heat may be generated in the course of the decay of the isotope to convert the powder into a molten form. In any event, it will be appreciated that heat-generating polonium isotopes may be introduced into the container 1 in any one of a number of different physical forms without going 3 outside of the scope of the invention.

In order to relieve the gas pressure on the interior of the container 1, due to the emission of alpha particles from the radioisotope contained therein, the cap 3 is provided with a helium permeable member or window 6 through which the helium is free to permeate but which prevents the isotope from escaping from the interior of the source. The gas permeable member 6 is supported on a flange 7 of the plug 3, and is cemented to the plug by a suitable adhesive S, such as a high-temperature cement or the like. In this manner, the permeable member 6 i: permanently secured to the cap, and becomes an integral portion thereof. The member 6 is characterized by the fact that helium and other gases having atomic diameters and atomic numbers of the same general order of magnitude as helium permeate through, while the member is impervious to solids, vapors and even gases having higher atomic numbers and larger atomic diameters. The importance of this latter fact, i.e., the selectivity in accordance with atomic numbers and diameters, can be appreci ated more fully when it is considered that certain alphaemitting isotopes, particularly in the radium, thorium, and actiniurn groups, decay to radioactive gases, such as radon, thoron and actinon, which radioactive gases would escape if a porous member generally permeable to all gases were utilized rather than the selective member which transmits helium and other gases in the general range of atomic numbers and atomic diameters of helium.

The term permeation as utilized in the instant application is defined as the overall steady-state flow process from the gas phase on one side of the window or membrane 6 to the gas phase on the other side. The term permeation therefore includes the diifusion mechanism which is the internal process by which an atom is handed on or changed from one lattice position to another within the window 6.

The total quantity of helium in cc. permeating the window 6 is given by the equation i-P2) d a; where K- the permeation velocity constant in cc. per second per cm. per mm. thickness per cm. of mercury.

A=the area of the membrane in cm.

t time in seconds.

P =the gas pressure in cm. of mercury on the high side of the window.

P the gas pressure in cm. of mercury on the low side of the window.

cl=thickness of the window in mm.

It has been found that the permeation velocity K of the helium through the window 6 is a function both of the composition of the window 6 and the ambient temperature. That is, the permeation velocity is a function of the percentage by weight of the glass forming oxides SiO B 0 and P 0 in the window 6. A number of glass compositions containing these glass-forming oxides have been found to be permeable to helium while impervious to gases of other types and a number of these, along with their percentage compositions, are given in the table below.

Glass Compositions Borosili- Soda 1 Sum ofpercent Glass Formers (SlOrI-BsOa-i-P205).

As pointed out above, the permeation velocity constant K in Equation 1 above depends both on the ambient temperature and on the composition of the material. FIGS. 2 and 3 illustrate this relationship graphically. Thus, referring now to FIG. 2, curves A through F illustrate the relationship of the temperature, plotted in deg. C. along the upper abscissa and as the reciprocal of the absolute temperature 1600/1, along the lower abscissa, for the various glass compositions tabulated above. Curve A of FIG. 2 illustrates the variation of the permeation velocity (log K) with temperature for a lead borate glass, curve B for an X-ray shield glass, curve C for a soda lime glass, and so on. These curves demonstrate clearly that the value of the permeation velocity constant K varies with temperature, so that the volume of gas permeating through the member 6 is higher at the higher temperatures. This, of course, is a most useful characteristic, in a radioi otope heat source, since the very heat generated by the isotope will aid in increasing the permeation velocity of the helium through the window 6.

In addition to its dependence on the temperature, the permeation velocity K is also proportional to the perentage of the glass formers SiO B 0 and P 0 in the window 6. FIG. 3 illustrates by means of the curve 9 the relationship of the permeation velocity constant K, as plotted along the ordinate, to the percentage by weight of the sum of these glass-forming oxides, for a given temperature of C. It will be observed that the permeation velocity K varies over several orders of magnitude as the composition of the member 6 changes from approximately 22% by weight of the glass-forming oxides to 100%. The curve 9 as pointed out above represents the relationship of the permeation velocity constant and the percentage of glass-forming oxide for a given temperature 100 C. Similar curves may be drawn for various other temperatures from the curves of FIG. 2

by plotting the permeation velocity constant K for each of the various glass compositions at the various temperatures plotted on the abscissa of FIG. 2. In any event, it is clear that the various glassy substances which have been found to be permeable to helium have varying degrees of permeability, depending both on the composition and the temperature at which the gas permeates therethrough.

It is apparent from the examples of the gas-permeable glasses described above that many other different glass compositions having varying percentages of the glassforming oxides may be utilized as the helium permeable member or window 6. Some typical examples of these are a glass composition marketed under the tradenarne Vycor, by the Corning Glass Works, which contain 96% of SiO 3% of B and 1% of A1 0 (99% of the glass-forming oxides); a glass composition marketed under the tradename Pyrex #7740, by the Corning Glass Works, which contains 81% SiO 13% B 0 2% A1 0 and 4% of sodium and potassium oxide combined. It is therefore impossible to attempt a comprehensive catalog of all the combinations and permutations of the glassforming oxides which may be fabricated and which may be useful in the particular device described in the instant application. The invention lies in the recognition that glasses characterized by the fact that they contain various percentages of the glass-forming oxides SiO B 0 and P 0 may be utilized in an isotope heat source as a member which is permeable to helium and other gases having atomic numbers and atomic diameters on the same order of magnitude, while yet being impervious to the radioisotope in its solid, vapor, or gaseous form. Furthermore, it will be understood that many other glass compositions may be formulated, by those skilled in the art, now or in the future, which will fall within the teaching contained in the instant application, and hence makes any attempt to include a comprehensive catalog of little or no use, and those skilled in the art are better served by means of the exemplary listing described above.

In describing the construction and characteristics of the novel radioisotope heat source illustrated in FIG. 1, the invention has been described in terms of the device utilizing a radioisotope heat-generating means in its molten form. It will be apparent, however, that the radioisotope may be utilized in its solid form, and FIGS. 4 and 5 illustrate such an alternative embodiment. Thus, in FIG. 4, a radioisotope heat source is illustrated in which a radioisotope is distributed in powdered form throughout a glass matrix 11 which is permeable to the helium generated by the emission alpha particles from the powdered isotope 10. The entire isotope-and-matrix component is positioned in a stainless steel housing 12 which has a circular opening 13 to permit the permeating helium to escape. The glass matrix 11 is again composed of a material containing a desired percentage of the glassforming oxide SiO B 0 and P 0 and in a preferred embodiment is fused quartz, which is 100% fused SiO and which has a relatively high helium permeation velocity constant K. The gas permeable matrix 11 is fabricated in such a manner that the isotope Polonium-ZlO, discussed above, is dispersed through the body of the glass.

The radioactive source illustrated in FIGS. 4 and 5 may be fabricated by dispersing the radioisotope in powdered form in a powdered glass frit suspended in a suitable carrier such as water, turpentine, or xylene. One suitable commercial form of powdered glass frit is manufactured and sold by the Ferro Corporation, Cleveland, Ohio, and is identified by their trade designation as #1028 Enamel Frit. The intermixed radioisotope and powdered glass flit in the liquid carrier is then placed in an oven and baked to a temperature of about 850 C. for a sufiicient period of time to allow the glass frit to melt and fuse into a glazed block having the radioactive material dispersed therein in the manner shown. The baking of the glass frit and the radioisotope may take place directly in the metallic container 12, or may be done separately, and the fused member may subsequently be inserted into the housing member in any suitable fashion, to provide the desired heat transfer surface for the heat energy generated by the emissions from the radioisotope dispersed in the fused matrix member.

While a particular embodiment of this invention has been shown, it will of course, be understood that it is not limited thereto, since many modifications in the arrangement and construction may be made. It is contemplated by the appended claims to cover any such modifications as fall within the true spirit and scope of this invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

l. A heat energy source of the type utilizing a radioactive isotope to generate heat during decay comprising retaining means for retaining a heat-generating alphaparticle-emitting radioactive isotope, means for relieving gas pressure on said retaining means and preventing release of the isotope in any physical form including a member permeable to the helium gas generated by the alpha-particle emission but impervious to the isotope in gaseous, liquid or solid form.

2. In a radioisotope heat source, the combination comprising means for retaining the heat-generating alphaemitting radioisotope, said retaining means including a helium permeable member for relieving the pressure due to the helium gas generated by the emitted alpha particles and preventing release of the radioisotope in any physical form, said member being impervious to the radioisotope in any form, and having at least one of the glass-forming oxides as a constituent.

3. In a sealed radioisotope heat-generating source, the combination comprising a closed container for retaining the heat-generating, alpha-emitting radioisotope, a permeable window in said container through which the helium generated by emission of the alpha particles perme ates to relieve the pressure on said housing, said window being impervious to the radioisotope in any physical form, the composition of said window including between 22 and percent by weight of glass-forming oxides.

4. In a radioisotope heat-generating source, the combination comprising a sealed container containing a heatproducing alpha-particle-emitting isotope, a gas permeable member through which the helium produced by alpha emission permeates to relieve the pressure on said container, said member being impervious to the isotope in its solid, liquid or gaseous form, said member being constituted of fused quartz.

References Cited in the file of this patent UNITED STATES PATENTS 1,505,209 Leach et al. Aug. 19, 1924 2,405,026 Feuer et a1 July 30, 1946 2,479,882 Wallhausen et a1. Aug. 23, 1949 2,491,370 Forster Dec. 13, 1949 2,651,730 Linder Sept. 8, 1953 2,873,853 Burton Feb. 17, 1959 2,883,553 Birden Apr. 21, 1959 

1. A HEAT ENERGY SOURCE OF THE TYPE UTILIZING A RADIOACTIVE ISOTOPE TOGENERATE HEAT DURING DECAY COMPRISING RETAINING MEANS FOR RETAINING A HEAT-GENERATING ALPHAPARTICLE-EMITTING RADIOACTIVE ISOTOPE, MEANS FOR RELIEVING GAS PRESSURE ON SAID RETAINING MEANS AND PREVENTING RELEASE OF THE ISOTOPE IN ANY PHYSICAL FORM INCLUDING A MEMBER PERMEABLE TO THE HELIUM GAS GENERATED BY THE ALPHA-PARTICLE EMISSION BUT IMPERVIOUS TO THE ISOTOPE IN GASEOUS, LIQUID OR SOLID FORM. 